Water, probably the most important liquid on Earth, is also one of its most unusual. While water has many special features, in this writeup, I will discuss a rather basic one: its density. The density is the mass per unit of volume. This mass is on Earth strongly linked to its weight, to the extend that may non-physicists use them interchangably. Technically, the weight is the force exerted by the mass in a gravity field.

Almost all substances contract when they cool and expand when they heat up. This effect is very well-known in gases, where the ideal gas law predicts that the volume of a certain amount of gas increases linearly in the temperature. The reason for this is that the speed and kinetic energy of the gas molecules increases with increasing temperature. Because the gas molecules knock harder to their surroundings, the gas will expand, lowering the density. In fact, one could say that the density of the kinetic energy remains constant.

For solids and liquids, a similar principle applies. If a solid or liquid heats up, the molecules vibrate more, and this expands the solid. Of course, the molecules in a solid or liquid are packed closely, so the expansion is a lot smaller than in a gas.

In water, the same thing happens. However, there is also a competing effect. Water has the ability to form so-called hydrogen bond. A hydrogen bond, very simply, is a kind of "bond" between the electrons of a strongly electronegative atom such as fluorine, nitrogen or oxygen and a hydrogen atom of another molecule 1. These bonds are very common in water, as each water molecule has one oxygen atom with two electron pairs and two hydrogen atoms available for hydrogen bonds. It is the molecule with the most hydrogen bonds compared to its size. Now, these hydrogen bonds are not quite as strong a "normal" bonds, but still a lot stronger that the normal attractive forces between molecules.

Great, but what does this have to do with density? Well, the hydrogen bonds force ice into a crystal structure that is not particularly dense. In fact, the density of ice is lower than that of water. This means ice floats on water. This is very rare: for almost all matter, the solid sinks in the liquid. This has important consequences for the freezing of water. When the air above a pond reaches 0 C, its top freezes, forming a solid layer. This layer "isolates" the rest of the pond from the cool air. In almost any other substance, the chunks of solid would drop to the bottom, and new liquid would surface, also freezing, leading to much more efficient and complete freezing. In the case of water, this means that ponds will almost always have liquid water at the bottom in which life can survive

It also has another consequence. The increased formation of hydrogen bonds at lower temperature means that when water approaches the temperature at which it becomes ice, it also starts to expand. Hence, water reaches its maximum density of 999.9720 kg m -3 at a temperature of 3.98 C 2, 3. This density is remarkably close to 1 gram per cubic centimeter of water - in fact, this is how it historically was defined 4. However, this is a very inconvenient definition if one wants do do accurate measurements - first of all, the temperature has to be precisely 4 C and, secondly the water has to be pure. Furthermore, the water has to be stored in a vessel, the mass of which must be subtracted form the mass of the water. Nowadays, the kilogram is defined using a block of metal in Paris, which is a much more accurate way of defining it. So, now you know it - the fact water almost has a mass of 1 gram per cubic centimeter is not a coincidence, but it is not exact, either.

In summary, the weight, or rather, the density of water is a complex function of temperature. In particular, water is most dense at around 4 C. This has important consequences for the way water behaves on Earth, and, in particular, for life. For a while water was used to define the gram; this is no longer done, as the result is inaccurate. However, this does mean that, conveniently, 1 cubic centimeter of water weighs 1 gram.


  1. http://www.elmhurst.edu/~chm/vchembook/161Ahydrogenbond.html
  2. http://www.simetric.co.uk/si_water.htm
  3. http://en.wikipedia.org/wiki/Water_(molecule)
  4. http://en.wikipedia.org/wiki/Kilogram

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